Development and Characterization of Carbon Nanotubes for Sensor Applications

The aim of this research was to develop, characterize, and analyze carbon nanotubes as biosensors. In particular, pH and lactate molecules were targeted in this study. The reason these analytes were chosen was twofold. Firstly, when hydrogen ions and lactate are excreted in abnormal amounts in human sweat, they may be an indicator of a separate health problem. Thus, there is a clinical need for such biosensor applications. Secondly, pH and lactate detection represent two different types of electrochemical sensing techniques. The carbon nanotubes used in this research were single walled and existed in bundles. They were further functionalized with the carboxyl group to detect pH and the enzyme lactate oxidase (LOX) to detect lactate. All carbon nanotube samples were characterized to compare the materials with the attached biomolecules and without the presence of biomolecules. Fourier Transform Infrared Spectroscopy (FTIR) was used to verify the attachment of both the carboxyl group COOH and LOX to the respective carbon nanotubes samples. Scanning electron microscopy (SEM) was used to analyze the carbon nanotube lactate electrode sample to examine the structure of the electrode. Both pH and lactate biosensors were used in a standard three electrode electrochemical cell where the carbon nanotubes behaved as the working electrode with an Ag/AgCl reference electrode and a platinum wire as the counter electrode. Each sample was separately interrogated by several voltammetry techniques such as linear, cyclic, and square wave. Square wave voltammetry proved to be the best template to use to sense the target analytes. The functionalized CNT-COOH electrode displayed a linear response to pH 1-10, with a negative voltage shift corresponding to an increase in pH. Two types of lactate sensors were fabricated, both of which exhibited an increase in current corresponding to an increase in lactate concentration. The functionalized CNT-LOX on a glassy carbon electrode displayed an amperometric response in the range of 1 mM - 4 lactate. The CNT-LOX on a Si/ITO substrate displayed an amperometric response in the range of 0.01 M - 0.05 M lactate